With increase of traffic, in an optical communication system, a wider bandwidth of a line and a higher functionality of a network are desired. Therefore, techniques such as OADM (Optical Add-Drop Multiplexer) and ROADM (Reconfigurable Optical Add-Drop Multiplexer) are applied to the optical communication system. Optical communication systems that use OADM or ROADM are disclosed in PTL 1 and PTL 2, for example.
In OADM and ROADM systems, WDM (Wavelength Division Multiplexing) communication is used and a transmission device inputs a client signal into a submarine cable as a wavelength multiplexed optical signal, and a plurality of paths are accommodated in a single optical fiber to improve flexibility of the network, for example.
In OADM and ROADM systems, an optical transmitter generally makes an optical control signal for controlling an optical relay device be included in a wavelength multiplexed optical signal to control the optical relay device. Further, an optical relay device may include an optical control signal in a wavelength multiplexed optical signal in order to indicate a state of the optical relay device.
PTL 3 discloses a technique in which a wavelength multiplexing transmitter multiplexes a plurality of optical signals containing data and a monitoring optical signal (control signal light) containing information for performing an adjustment or the like of a gain of an optical relay device and transmits the wavelength multiplexed signal to an optical fiber. The wavelength multiplexing transmitter described in PTL 3 includes a plurality of light sources which output a plurality of respective optical signals and a monitoring light source which outputs a monitoring optical signal (control signal light), and generates a wavelength multiplexed signal by multiplexing the plurality of optical signals output from the plurality of light sources and the monitoring light source.